This invention relates to perimeter intrusion alarm systems, and more particularly to such systems using microwave energy transmitted and reflected to a monostatic antenna for motion detection.
Intrusion detecting devices and systems utilizing microwave energy are known. Many such devices and systems utilize transmission antennas and reflectors positioned to define a perimeter of interest. Such a series of antennas and reflectors requires elaborate installation and accurate alignment to insure reasonably reliable intrusion detection. Heretofore, many presently available systems did not provide the expected detection reliability because of the ease with which an intruder was able to compromise the system. The detection reliability of presently available systems is further reduced by the effects of ambient conditions that cause a disruption of a transmitted energy wave.
A feature of the present invention is to provide a perimeter intrusion alarm system utilizing a flexible distributed antenna. This flexible antenna is relatively easy to deploy around any given perimeter segment of interest. Operation of the antenna insures that the energy therefrom is confined to a narrow zone about the protected perimeter segment, thereby permitting authorized activities beyond the protected zone without causing false alarms. As a further feature of the flexible distributed antenna, detection is extended across perimeter gaps (such as open gates, driveways, etc.) by means of end lobe energy transmitted from the antenna.
Another feature of the present invention is to provide a perimeter intrusion alarm system wherein a flexible distributed antenna is connected to a transceiver providing balanced quadrature mixing. Two doppler frequency outputs from the mixer are processed into single sideband "approach" and "recede" signals to reduce the false alarm rate. These "approach" and "recede" signals are processed through a voltage comparator to trigger an alarm indicator when the differential of the two signals exceeds a given level.
Still another feature of the present invention is that actual radiated energy flow is set up perpendicularly to the direction of the perimeter area of interest. Hence, an intruder cannot approach the protected segment without generating a true doppler signal. By contrast, certain other prior art systems (Gaubau lines, etc.) utilize a guided energy flow moving along the direction of the perimeter segment. Consequently, only components of intruder motion parallel to the perimeter segment generate true doppler signals. The implication is that a knowledgeable intruder has a better chance of defeating systems of this prior art type.
Since false alarms tend to be the greatest single problem in perimeter alarm systems, yet another feature of the present invention is the processing technique for reducing the false alarm rate. An approaching or receding intruder of the perimeter segments produces a single sideband return either above or below a transmitted frequency. False alarm sources, such as blowing trees, vibrating fences, etc., tend to produce double sideband returns and this single sideband technique is utilized to enhance the legitimate intruder signals. However, provisions are made to alarm on abnormally high noise levels produced by these ambient conditions to prevent "jamming" by an intruder.
One class of motion detection systems employs a sensitive receiver in conjunction with a transmitter to receive and measure an electric field. If an intruder or foreign object disturbs the electric field, there results a variation in the field strength which is detected by the receiver and used to trigger an indicator or alarm system. Another class of motion detection systems is the space alarm system characterized by the transmitting of energy into a specified space to be protected, or the space surrounding an object to be detected, and subsequently receiving that portion of the transmitted energy that is reflected by the surroundings. An alarm is triggered upon the detection of a disturbance, i.e., a frequency change, in the reflected energy caused by an intruder within the area. Any frequency change of the reflected energy, as compared to the transmitted energy, will indicate an object is moving within the area being monitored. This is the principle of operation of the well-known "doppler" effect. This type of system detects a doppler frequency shift in radiation reflected by moving objects within a specified area and the present invention utilizes the "doppler" effect.
A drawback of many microwave doppler type of intrusion detectors is the need for expensive and complex systems utilizing separate transmit and receive antennas for protection of extended areas or, if a single antenna is used, expensive duplexor networks are used to protect the receiver from damage by the high power signals being transmitted. The microwave doppler system of the present invention utilizes a single transmit/receive (monostatic) antenna in conjunction with miniaturized RF circuitry to provide balanced doppler mixing without complex radio frequency devices.